1. Field of Invention
The present invention relates to a method for determining the quality of a motor and, in particular, to a method for discerning the quality of a rotor by winding an induction coil on a stator and measuring the electromotive force of the motor.
2. Related Art
Currently, development in information recording media aims at high-density data storage density, data transmission speed has to become faster too. Correspondingly, the rotational speed of the main-axis motor on the storage device such as an optical disk drive or DVD has to be able to satisfy such a requirement. Therefore, the quality and characteristic of the main-axis motor are very important factors.
One factor that has great influence on the motor characteristic is the back-emf constant Ke of the motor. In the MKS system, it is numerically identical to the torque constant Kt. Since the back-emf Ke is a load for the external voltage applied to the motor, the external voltage V has to be greater than the back-emf so that the motor can function in the form of a motor. Otherwise, it would be running like an alternator. One then knows that only the difference between the external voltage and back-emf can provide a current on the motor coil and have a torque output. Furthermore, one intuitively thinks that a motor with a large Kt value can obtain a constant torque with an extremely small current. However, the Ke value also increases. That is, a tiny rotational speed can make the back-emf greater than the external voltage. Under such a condition, the achieved rotational speed is certainly low and unsatisfactory. Thus, the value of Ke actually determines the rotational speed and the torque character of the motor.
There are two major methods of measuring the value of Ke. Referring to FIG. 1, the first method is to use an active motor 11 to drive a test motor 12 into rotation so as to measure the back-emf of the stator in the motor 12 (Eb in the drawing) and thus the Ke value. Although this method can have fairly accurate results, the driving of the active motor 11 on the motor 12 is mediated through a connection axis. Thus, there is the problem of axis alignment. If improperly manipulated, the connection axis will affect the measurement and do harm to the rotational axis of the motor 12. Moreover, the industry can not test the motor rotor immediately after it is made. The rotor can be tested only after a set of motors 12 are assembled. If there is any problem, the whole set of motors 12 has to be thrown away and invested product line equipment has to be adjusted. Since an active motor 11 is employed to drive test motors 12, the testing costs more time.
Please refer to FIG. 2. Another conventional method is to measure a single test motor rotor 22 using a Gauss meter 21 to obtain the magnetic flux density B. Yet this method is not measuring the result of a rotor and stator system under operation, the obtained result will be quite different from the actual situation. That is, it is impossible to measure the back-emf generated due to the magnetic force line crossing between the rotor and the stator.
In view of the foregoing, it is an object of the invention to provide a method for determining the magnetic induction of a rotor along, whose result is not different from directly measuring the motor under actual operation. This method can control the stability of the motor quality.
Pursuant to the above object, the invention provides a method for discerning whether a rotor is good in magnetic induction by measuring the electromotive force of the motor. According to the disclosed technology, a motor standard stator is manufactured to be the standard of all test rotors. In addition to driving coils, the standard stator is further coiled with a set of induction coil on its teeth. When a user wants to test the quality of a rotor, he only needs to combine the test rotor with the standard stator. A driving voltage is provided to the driving coil through a driver to rotate the rotor. Through the induction of the induction coil, the back-emf signal generated by the test rotor is given out. The back-emf signal is also retrieved and converted into a physical signal related to the motor rotational speed. By computing the back-emf signal, the physical signal and the ratio of coil rounds on the driving coil and the induction coil, the back-emf constant of the motor can be obtained. Therefore, the disclosed method can discern the quality of motors.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.